The present invention relates to decoding of telecommunication signals, and more particularly to despreading of modulated signals that are spread by a plurality of spreading codes for effective decoding with low noise.
IMT-1000 is an International Telecommunication Union (ITU) standard for third generation (3G) mobile telecommunications which encompasses cdma2000, UMTS (Universal Mobile Telecommunications Systems) and W-CDMA (Wideband—Code Division Multiple Access). The CDMA spread spectrum communication method is being commonly adopted by next generation mobile phone systems. FIG. 1 shows a block diagram of a base station (transmitter side) 10 that uses a plurality of spreading codes according to one of the 3GPP (Third Generation Partnership Project) standards. For simplicity, FIG. 1 shows only two channels being assigned to one spreading code, but actually 256 or more may be assigned, and the total number of channels determines the maximum number of mobile phones that the base station can support. Each mobile phone may use not only one, but a plurality of channels. If too many spreading codes are used at the base station, it increases inter-symbol interferences that make demodulation difficult. Therefore only a few spreading codes are used at each base station. The above system has an advantage in that many more channels are available at each base station relative to a system that assigns one spreading code to each base station.
Channels CH1-CH6 (21-26) in FIG. 1 provide respective digital modulated signals that are initially data modulated according to some modulation method, such as QPSK (Quadrature Phase Shift Keying), etc. The digital modulated signals are multiplied by Walsh codes (C1-C6) in respective multipliers 31-36 to make their relationships orthogonal to each other. Then adjacent Walsh-coded channel pairs are summed at respective adders 42-46, i.e., the output signal from adder 42 is a sum of digital modulated signals from channels CH1, CH2, hereinafter called “a digital modulated signal corresponding to spreading code A”. The addition process does not lose any information of the digital modulated signals from channels CH1, CH2 since they are orthogonal as a result of the Walsh code multiplication. A multiplier 52 applies spectrum spreading on the sum of the digital modulated signals of channels CH1, CH2 using spreading code A. Similarly, spreading codes B, C are applied to the respective signals of other channel pairs by respective multipliers 54, 56. The resulting spread signals A, B, C are summed by an adder 58. The output signal from the adder 58, or the sum of the spread signals, is modulated by a carrier signal (60) and transmitted through an antenna 62. As described above, each base station conducts a multi-modulation process including multiplication by Walsh codes and spreading codes before the signal transmission.
The spreading codes are code series that are not relevant to the data (digital modulated signal) in the channels, and that spread the frequencies of the digital modulated signals of the channels to enhance information confidentiality and interference immunity. In the case of W-CDMA there are 8192 different spreading codes to which serial numbers 0-8191 are assigned. The spreading codes assigned to each base station for mobile phones are selected from the 8192 codes so as not to select the same codes as are assigned to adjacent base stations.
FIG. 2 shows a block diagram for a conventional receiver 70, such as contained in a mobile phone. An antenna 72 receives the transmission signal from the base station 10 and a detector 74 detects the sum of the spread signals from the received signal. A correction circuit 76 corrects sync, frequency and phase and extracts one unit necessary for demodulation. A despreading circuit 82 demodulates from the received signal a digital modulated signal including a desired channel corresponding to a spreading code. If the desired channel is channel CH2, a digital modulated signal corresponding to spreading code A is demodulated. The digital modulated signal from the despreading circuit 82 is the sum of digital modulated signals from channels CH1, CH2. A channel separator 84 separates the channels according to the applicable Walsh code. Then the mobile phone or terminal gets the digital modulated signal for the desired channel.
The despreading circuit 82 has to decide which spreading codes from among the 8192 codes are used. The IMT-1000 standard defines that the received signal includes pilot signals and information that indicates spreading code numbers, and one of the spreading code numbers is used. Then the despreading circuit 82 tries the spreading codes, corresponding to the numbers deemed to be used based on the spreading code information, one by one to determine whether the spreading code decodes the pilot signals normally in order to identify the spreading code used in the received signal. The spreading code identification process starts just after receiving the signal and is over before a conversation starts. When the base station uses a plurality of spreading codes, they have known offsets between them so that, if one spreading code is identified, other spreading codes are automatically determined.
The receiver 70 of FIG. 2 is simple and does not require much time for data processing, so it is popular when real time processing of the received signal is required, such as in a mobile terminal or phone. The spreading codes are selected to have a low correlation to each other—most of the signal components not corresponding to the desired spreading codes are eliminated.
However the spreading codes are not completely orthogonal to each other, so components of inter-symbol interference remain as noise to some extent. The noise components may cause problems when communication data rates increase and improved quality is required for the decoded signal. A measurement instrument is required to recover the original signal accurately, so it needs to eliminate the noise components to a maximum extent. The circuit shown in FIG. 2 does not provide the required level of noise elimination.
U.S. Pat. No. 5,579,304 discloses a conventional method of reducing the inter-symbol interferences when signals are received from a plurality of base stations. However the method uses cascaded interference reduction stages having complicated circuits.
As described above, some inter-symbol interferences remain since the spreading codes are not completely orthogonal. It is not a critical problem for a mobile phone as long as conversation is possible even if there is some noise. However, some applications require an accurate original signal recovery and noise is an important problem. Therefore, what is desired is to demodulate a signal before spreading while effectively eliminating inter-symbol interferences during the despreading process.